Gradient texture not showing up in rendered view
Shaders

A shader is the function that describes the lighting interaction at the surface or of the volume. That is different from the color of the surface itself.

A red carpet's surface is still of the same shade of red even in a dark room, we just perceive it dark because there is no light around.

At the moment you are defining only the "color property" (yellow socket) of the surface, but not how this property should interact with light. You shoud use one of the nodes you can find under the "shaders" menu. It will acquire the color data as input and output a shading data (green socket) that should be connected to the Surface's socket of the MAterial output.